An aperture antenna such as a horn antenna has a waveguide portion with its cut-off wavelength being smaller than that of outgoing electromagnetic wave and a flare portion with its diameter being enlarged as approaching an aperture for impedance matching with space.
FIG. 1 depicts a conventional horn antenna 11. The conical horn antenna 11 has a circular waveguide 12, a flared conical portion 13 connected to the circular waveguide 12, and an oscillation source 15 to supply power. The circular waveguide 12 has a cut-off wavelength of 3.41A/2 in TE11 mode, where “A” being the inner diameter of the circular waveguide 12. Any electromagnetic wave with shorter wavelength than the cut-off wavelength does not pass through the circular waveguide 12. Assuming that an outgoing electromagnetic wave has a wavelength lambda, the inner diameter “A” of the circular waveguide must be greater than 2*lambda/3.41, or A>=2*lambda/3.41. The flared conical portion 13 connected to the circular waveguide 12 has inner diameter enlarged as approaching the aperture 14, with the inner diameter at the aperture 14 being approximately lambda. This matches the impedance of the antenna with that of space.
FIG. 2 depicts another conventional horn antenna 21. The conical horn antenna 21 illustrated in FIG. 2 has a metal coaxial line 26 as well as a circular waveguide 22 and a flared conical portion 23 which are similar to those of the conical horn antenna 11 illustrated in FIG. 1. The metal coaxial line 26 allows TM11 mode to be generated in the circular waveguide 22, and as a result, the circular waveguide 22 has grater cut-off wavelength than the circular waveguide 12. The flared conical portion 23 has greater inner diameter as approaching an aperture 24. The metal coaxial line 26 protrudes by length “C” from the boundary line between the circular waveguide 22 and the flared conical portion 23.
It is difficult to make a conventional horn antenna compact due to the flared portion provided therein.
In addition, it is known in the art of plasma generation apparatuses a slot antenna for radiating microwave with a center conductor being protruded.    [Patent Document 1] Japanese Laid-open Patent Publication No. 11-284428    [Patent Document 2] Japanese Laid-open Patent Publication No. 2004-266268